Shell colour and pattern in molluscs have long been considered fixed properties, set down as the shell is built. Researchers at the University Museum of the University of Tokyo have overturned that assumption, showing that two unrelated tree snails, Hypselostyla camelopardalis from the Philippines and Reinia variegata from Japan, independently evolved dynamic camouflage. When wet, their light mottled patterning disappears and the shell turns uniformly dark, resembling rain-soaked bark; the pattern returns as the shell dries.
The mechanism lies in the periostracum, the shell's outermost organic layer, which is sponge-like with pores at the nano- to microscale. Water filling these pores reduces light scattering through refractive index matching, letting light pass through to dark pigments in the crystalline layer beneath. Measurements showed transmittance rising from roughly 37 percent when dry to about 85 percent when wet.
The study, published in Zoological Letters and built on field-emission scanning electron microscopy, confocal laser imaging and spectrophotometry of specimens held in the university's own collections, is also an elegant case of convergent evolution: two distant lineages arriving at the same optical solution independently. Because the effect requires no energy input from the animal, the authors suggest the mechanism could inspire humidity-responsive smart materials and passive environmental or medical sensors.